By Robert Deyes
What should we truly expect from the human lineage? What should ancestral species look like on an evolutionary road driven by the slow turning of the wheels of natural selection? Surely if we subscribe to the idea that at the base of our intellectual richness existed a primitive 'brute', as Darwin's 'bulldog' Thomas Henry Huxley once quipped (Ref 1), then at least brain size should be one aspect of our anatomy that we would expect to increase with time? With such expectations, we can truly understand the confusion that arose from the finding of a 12,000 year-old, tool-bearing species of Homo in the Indonesian island of Flores (Portuguese for 'flowers') that would have had a brain size equal to that of the smallest early hominins (Refs 2-3). The television documentary Alien From Earth referred to the new specimen as "an elf like creature with over-sized feet" (Ref 4).
In his controversial paper Peter Brown, together with his colleagues from the University of New England in Australia, described the Flores specimen as a small bipedal man that displayed, "endocranial volume and stature...similar to, or smaller than, Australopithecus afarensis" (Ref 3). Its tiny brain was smaller than that of a chimpanzee's and only one third the size of a human's (approximately 400 cubic centimeters, compared to 1350 cubic centimeters seen on average in living humans, Refs 2,5,6). One Scientific American report gave the following description of this extraordinary find:
"The specimen appears to have belonged to an adult female who stood barely a meter tall and had a skull the size of a grapefruit--the smallest member of the human family yet. Although closer in overall size to the much older australopithecines...the new hominid apparently resembles members of the genus Homo in features related to chewing and upright-walking." (Ref 7)
Touted by the same report as, "one of the most spectacular paleoanthropological finds of the past century", H. floresiensis, or 'the hobbit' as it is now affectionately known, is believed to have existed between 95,000 and 12,000 years ago (Refs 5,7). Twelve individuals have so far been unearthed in the remote cave of Liang Bua alongside items that demonstrate that they would have been skillful tool makers. (Refs 2,4,8). Archaeologist Carol Lentfer believes the tools of the Flores site might have been used to "craft spear shafts of wood or bamboo or items like traps- a tool kit for making other tools" (Ref 5). Others contest the idea that these tools were really that sophisticated preferring to see them as nothing more than "simple stone artifacts" (Ref 5).
Placed into the overarching view of human evolution, the brain size of H. floresiensis does not fit into the expected sequence of evolutionary intermediates (Ref 4). In fact some experts have claimed the H. floresiensis challenges the idea that bigger brains and bodies were necessarily the critical elements that allowed humans to colonize the world (Ref 4).
Skeptics have taken Peter Brown to task by inferring that the small heads of H. floresiensis were nothing more than than the bi-product of microcephaly- a disorder that produces the shrinking of the brain (Refs 6,8). Alan Thorne from the Australian National University for example commented that H. floresiensis was a 'pathological specimen'- a human individual adversely affected by serious disease (Ref 10). But anthropologist Dean Falk has compared the brain of H. floresiensis and microcephalics through Computed Tomography and found dramatic differences between them (Ref 11). Based on their own findings on the relative proportions of the head and body size, Peter Brown's team likewise dismissed the assertion that the small size of H. floresiensis was merely a manifestation of some growth disorder (Refs 2,3,12). Nevertheless their own alternative postulates appeared to be nothing more than speculative musings about how selective pressure might have contributed to the small size of these island people . Indeed they conclude that,
"the location of these small hominins on Flores makes it far more likely that they are the end product of a long period of evolution on a comparatively small island, where environmental conditions placed small body size at a selective advantage" (Ref 3).
How long such a period of evolution would have needed to be is today a matter of deep debate (Ref 6). John de Vos and his team from the National Museum of Natural History in the Netherlands have been outspoken in their view that H. floresiensis evolved from the much larger H. erectus and shrunk once it took on the challenges of island living (Ref 12). According to de Vos strange things happen on islands. With few predators and limited resources animals often grow or shrink and this is what might have happened to H. floresiensis (Ref 12). To buttress his arguments de Vos has cited well-documented cases of giant island rats and pygmy elephants (Ref 5). And yet like most other aspects of the Flores debate, such 'island dwarfing' is not without its own doubters (Ref 2). Indeed what distinguishes H. floresiensis from other cases is the extent of brain shrinking that would have followed from, say, a much larger-brained H. erectus- a point that, in the eyes of Field Museum paleoanthropologist Robert Martin, represents a major stumbling block for protagonists of the dwarfing hypothesis (Ref 5).
Stony Brook University's Bill Jungers and Australian National University's Debbie Argue assert that ancestral links go back much further than H. erectus to perhaps two million year old H. habilis or even three million year old Australopithecus (Refs 4,5,8,9). The implications of such a position are enormous since it suggests that larger brains and body sizes were not necessarily the outstanding features that lead to the conquest of our planet by humans (Ref 4). Moreover, an Australopithecine origin would invoke a period of approximately 3 million years of undetected evolution between H. floresiensis and his first known ancestors.
Where does the Flores debate stand today? The most recent anatomical results are being used to embrace the idea of a deep evolutionary origin (Ref 2, 9). The island dwarfing hypothesis has also gained traction following discoveries of dwarf hippos on Madagascar that display notable differences in brain size from those of their mainland counterparts (Ref 2, 13). If the hippo scaling model is applied to a hypothetical African H. erectus, an endocranial capacity close to what we see in H. floresiensis could be achieved (Ref 13). That at least is how London Natural History Museum scientists Eleanor Weston and James Lister have interpreted their most recent findings (Ref 13).
Regardless, what is becoming clear from these studies is that in many aspects of its anatomy H. floresiensis presents us with a clear 'misfit' for the human evolutionary sequence. In the words of one review "[H. floresiensis] threatens to overturn our understanding of where we come from and the type of ancestors that have shared the human family tree" (Ref 4). Chief Nature science editor Henry Gee had this to say on the ongoing Flores enigma and the consternation it has caused amongst human anthropologists:
"Despite decades of patient work we still know rather little about the evolution of humanity...the remains we have are very scarce and very meager and that means that there are probably lots of different species that existed, lived for hundreds of thousands of years and then became extinct and we know nothing about them...All you need is just one to completely blow apart your well entrenched comfortable idea of the linear progress of evolution" (Ref 4).
While Gee's wild speculation over missing species seems undeserved of the title of objective science, his concerns do tell of a crisis for evolutionary biologists. In short, H. floresiensis has today become the flower that is shaking the human evolutionary tree. Findings such as these turn our cherished notions of human evolution upside down since they show tool-making, small-brained hominin species living alongside humans as recently as 12,000 years ago.
One 'escape chute'-style answer to this paradox assumes that specimens such as H. floresiensis are exactly what one would expect if nature were to be constantly experimenting with alternative evolutionary solutions in the face of rapidly changing selective pressures (Ref 4). Of course such an exit pre-supposes rather than demonstrates that all hominins are part of an all-encompassing lineage of diverging branches. Moreover, we still lack any understanding of how evolution might bring about the dramatic changes we see in any of the hominin remains (Refs 14,15).
1. Roger Lewin (1987), Bones of Contention: Controversies in the Search for Human Origins, Published by Simon and Schuster, New York, p.309
2. Daniel Lieberman (2009), Palaeoanthropology: Homo floresiensis from head to toe, Nature Vol 459, pp.41-42
3. P. Brown, T. Sutikna, M. J. Morwood, R. P. Soejono, Jatmiko, E. Wayhu Saptomo; Rokus Awe Due (2004), A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia, Nature Vol 431, pp.1055-1061
4. Alien From Earth, Aired On Wisconsin Public Television on the 29th of April, 2009, See transcript at http://www.pbs.org/wgbh/nova/transcripts/3515_hobbit.html
5. Elizabeth Culotta (2007), The Fellowship of the Hobbit, Science Vol 317, pp.740Ã¢â‚¬â€œ742
6. Rex Dalton (2009), 'Hobbit' was a dwarf with large feet, Nature, 6 May 2009, See http://www.nature.com/news/2009/090506/full/news.2009.448.html
7. Kate Wong (2004), Mini Human Species Unearthed, Scientific American, October 27th, 2004. Access article on http://www.sciam.com/article.cfm?articleID=000B7CEA-EA31-117E-AA3183414B7F0000
8. Andy Coghlan (2005), New "hobbit" bones bolster separate species claim, October 11th, 2005. See article at http://www.newscientist.com/article.ns?id=dn8128
9. W. L. Jungers, W. E. H. Harcourt-Smith, R. E. Wunderlich, M. W. Tocheri, S. G. Larson, T. Sutikna, Rhokus Awe Due, M. J. Morwood (2009), The foot of Homo floresiensis, Nature Vol 459, pp.81-84
10. John Vidal (2005), Bones Of Contention, The Guardian, 13th January, 2005, See http://www.guardian.co.uk/science/2005/jan/13/research.science
11. Dean Falk, Charles Hildebolt, Kirk Smith, M. J. Morwood, Thomas Sutikna, Jatmiko, E. Wayhu Saptomo, Herwig Imhof, Horst Seidler, and Fred Prior (2007), Brain shape in human microcephalics and Homo floresiensis, PNAS, Vol 104, pp.2513-2518
12. G.A. Lyras, M.D. Dermitzakis, A.A.E. Van der Geer, S.B. Van der Geer, J. De Vos (2008), The origin of Homo floresiensis and its relation to evolutionary processes under isolation, Anthropological Science, See http://www.jstage.jst.go.jp/article/ase/advpub/0/0807280043/_pdf
13. Eleanor M. Weston, Adrian M. Lister (2009), Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis, Nature Vol 459, pp.85-88
14. Robert Deyes (2008), Turbulent Times Amidst The Desperate Maneuverings Of Human Evolutionary Theory PART I, See http://www.arn.org/blogs/index.php/2/2008/07/12/turbulent_times_the_desperate_maneuverin
15. Robert Deyes (2008), Turbulent Times Amidst The Desperate Maneuverings Of Human Evolutionary Theory, PART II, See http://www.arn.org/blogs/index.php/2/2008/07/14/turbulent_times_the_desperate_maneuverin_1
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